Tuesday, April 26, 2016

On authorship...life in a mega-collaboration

Scientific collaborations have rapidly increased in size over the past 40 years, with particle and nuclear physics leading the way.    Groups of half a dozen researchers have given way to mega-collaborations comprising thousands of people - 2,800 authors in the case of the ATLAS experiment at the LHC.  

The enormous growth in size has led to inevitable changes.   Large organizations require structure; in the case of scientific collaborations, this includes written rules (governance documents), elections for leaders, usually called 'spokespersons' (to emphasize that their job is to represent the collaboration), committees, and more committees.  

These developments are driven, for the most part, by the demands of the science, which require large complex detectors, and ever more detailed analyses.   In many areas, large groups are required to make progress.  These developments raise a number of sociological questions.  To me, one of the more interesting questions is what it means to be an author in a large collaboration. 

Recently, I wrote  guest post for Retraction Watch, entitled, "When it takes a village to write a paper, what does it mean to be an author?" that goes into this question in more detail.  I may be biased, but I think it is worth reading.

If you haven't heard of it, Retraction Watch is a blog that covers retractions in the scientific literature, whether due to mistake or misconduct.  They also have interesting articles (and links) to pieces that discuss authorship and other ethical issues, including on the 'business' of science.

Monday, February 1, 2016

ARIANNA the 2015 field season

The 2015 ARIANNA season is over.  Actually, it has been over for a while, but I have been remiss in posting. 

Instead of me writing something based on secondhand discussion, I am just going to point you to the blog maintained by Anna Nelles, who was one of the three people who deployed this season:


Thursday, December 3, 2015

ARIANNA 2015/6

The ARIANNA 2015/6 season is well underway!

Three intrepid UC Irvine researchers are on the ice, working on upgrading the existing 7 stations, and also making more measurements of the radio-attenuation and radio-reflection properties of the ice in Moore's Bay.  They are scheduled to spend 23 days encamped, returning to McMurdo station in on Dec. 6th.

The main station upgrade consists of adding in small batteries which should make the stations work more smoothly during the 'shoulder' season (Spring and Fall), when the Sun is very low in the sky, and so the solar power can be somewhat erratic.  They are also investigating some problems to the solid state memories (SDD cards) which are likely associated with erratic power during these periods.

This being the new and modern era, they are communicating via twitter: @arianna_on_ice

Thursday, August 6, 2015

The most energetic neutrino - in picture form

This is an event view of the highest energy IceCube neutrino that I mentioned in my last post, as presented at the 2015 International Cosmic Ray Conference.  Each sphere is one optical sensor; the colored spheres show modules that observed light from this event.  The sizes of the spheres show how many photons each module observed, while the color give some idea of the arrival time of the first photon, from red (earliest) to blue (latest).

It is easy to see that the neutrino is going slightly upward (by about 11.5 degrees), so the muon cannot be from a cosmic-ray air shower; it must be from a neutrino.  The fact that the muon comes from just below the horizon is not surprising.  At PeV energies, neutrinos interact more than at lower energies, so the Earth is opaque.  So, we do not expect to see near-vertical upward-going neutrinos with PeV energies.

The measured visible energy is 2.6 +/- 0.15 PeV (1 PeV = 10^{15} eV).  This is the energy actually seen in the detector.  It does not include the energy lost by the muon before it reaches the detector,  the energy carried off by the outgoing muon, or the fraction of the neutrino energy that was transferred into a hadronic shower, rather than the observed muon.  So, the actual neutrino energy is a multiple of this. 

Wednesday, July 29, 2015

The most energetic neutrino yet!!

IceCube has just announced the observation of our most energetic neutrino yet.  The event was in the form of a through-going muon, which means that we saw a piece of the track in our detector, but the track both originated and ended outside of the enclosed volume.  So, we cannot measure the total energy of the neutrino.  Instead, we measure the specific energy loss (energy loss/distance, or dE/dx).  From that, we can estimate the muon energy, in the detector, and, from that, we can, based on an assumption of the neutrino energy spectrum, estimate the probability that neutrino had a range of energies.   We are still working on estimating the neutrino energy, but the total energy loss visible in the detector was 2.6 +/- 0.3 PeV.  This is, of course, a lower limit to the neutrino energy, making it clearly the most energetic neutrino yet observed.  Typically, one expects the neutrino energy to be a couple of times higher than the muon energy.

The event came from the Northern Sky (coordinates R.A.: 110.34 deg and Decl.: 11.48 deg), and we currently estimate the average angular resolution to be 0.27 degrees.   Because it was upward-going, we know that it must be a neutrino, and not cosmic-ray muon background.

The event was recorded on June 11, 2014 (see the link for details), just over a year ago.  We are clearly getting better at processing and analyzing our data more quickly, but there is still room for improvement.

The event was announced as an "Astronomers Telegram," (ATel) a brief announcement which can be issued quickly.  The main purpose of ATels is to get word out quickly about a new transient phenomena (gamma-ray burst, nearby supernova etc.), so that other astronomers can point their telescopes in right direction while the phenomena is still going on.  In our case, there is no reason to expect this to be (or not be) from a transient phenomena, and, if it was, it is probably over now.  But, we are releasing the coordinates now, so that other observatories can see if they can find anything unusual in that direction.   The event is relatively near the equator, so it should be visible from most large observatories.

More information later (probably after the Intl. Cosmic Ray Conference, July 30-Aug. 6th).

Note Added (July 30th).  Unlike Bert, Ernie and Big Bird, we have named this event after a Muppet. 

Wednesday, July 15, 2015

Getting Science Right

Every once in a while, a scientific scandal makes big news.  Someone faked doing an experiment, or grossly misinterpreted their results, or failed to reproduce someone elses important result.  Particularly in medicine, this can have big consequences.

Unfortunately, these scandals are often blown out of proportion, with insinuations that many scientists are dishonest.   At least partly because of this, scientists are paying increasing attention to irreproducible results.  There is a blog, "Retraction Watch" which is devoted entirely to scientific papers that have been formally retracted.  Some common problems are plagiarism (including self-plagiarism) or apparently faked results (particularly manipulated images).  Honest scientific mistakes (i.e. missed minus signs, etc.) also make an appearance, as does occasional subversion of the peer review process.    These problems are real, but it is important to keep them in perspective.  Retraction watch typically posts 1-3 retractions/day, out of hundreds of thousands of scientific papers published each year.  This is a very miniscule percentage.  Although Retraction Watch probably doesn't catch every retraction, they do appear to be very efficient at finding them.

Many of these errors are caught rather quickly, by other scientists.  Most important retractions occur within a year or two.   Pubmed, an online library of medical literature, run by the National Institute of Health recently (2013) opened Pubmed commons, where readers can comment on the scientific literature; suspect images and other visible problems can be (and are) vigorously discussed.  

A bigger problem may be papers that are just not reproducible, for reasons that are not clear.  This is mostly an issue for biology and medicine, fields that deal with complex systems (large molecules, cells, humans), where .   At least according to some reports, like this article in the New Scientist, this is an epidemic problem, affecting a large fraction of published papers.  This track record is a good reason to take the latest medical advice with at least a small grain of salt.  However, even here, the scientific record is generally self-correcting, albeit most slowly.  Science builds on previous results, and you can't build much on a cracked foundation. Darwinian evolution gradually weeds out bad conclusions.

As a more quantitative science, physics suffers less from irreproducibility than biology.  It is far easier to quantify the uncertainties in a neutrino energy measurement than in, for example, unknown contaminants in a reagent used in a biology experiment.    Over the past decades, physics has also taken increasing efforts to eliminate sources of unconscious bias.  In many experiments (IceCube included), most analyses are done in a 'blind' manner, whereby the analyst prepares his analysis using simulated data, and a small fraction of the real data.  Only after the analysis procedure is fixed, and reviewed by the collaboration, is the real data analyses.  This avoids any tendency to zero in on fluctuations in the data (the 'look here' phenomena).  So, when choosing a list of possible neutrino sources to analyze, we won't unconsciously pick one(s) that correspond to upfluctuations in the data.   As a result of these practices, particle and nuclear physics have a pretty good (but not perfect) record with being able to reproduce previous results.

Wednesday, March 4, 2015

Bert and Ernie's less energetic cousins

Since the original observation of Bert and Ernie, followed by Big Bird, we have been trying to learn all we can about our extra-terrestrial neutrino signal.   As previously mentioned, we have seen no statistically significant sign of any clustering indicative of a single source or multiple sources.  So, our efforts have been focusing on ways to better characterize the signal.   Today, I want to tell you about two efforts.

One obvious way to progress is to study the signal at lower energies (at higher energies, we don't see anything).   This is, however, easier said than done, since the backgrounds rise steeply at lower energies.   One way to handle that is to use larger and larger veto regions as the energy drops;  this leaves a smaller and smaller fiducial (active) detector volume, but the signal should also rise at lower energies.  Jakob Van Santen pursued this route, in an IceCube paper that was published in Phys. Rev. D. in January, and available on the Cornell preprint server, as arXiv:1410.1749.    

The plots below show the results of that study, for the Northern and Southern skies respectively.   The backgrounds get larger at lower energies.  In the Southern sky, there are two types of backgrounds: penetrating muons and an irreducible background of atmospheric neutrinos, while, in the North, only the neutrinos are present.  However, in both cases, it is possible to measure the astrophysical component down to deposited energies of a few TeV.   Here, "deposited energy" means the energy visible in the detector. 
The measured flux is consistent in both hemspheres, and is well fit by a power-law spectrum:                      phi ~ (E_nu)^-p, where p, the power law index, is 2.46 +/-0.12.  The spectrum is significantly softer (i.e. has more low-energy events and fewer high-energy events) than the standard benchmark spectrum, which is dN_nu/dE_nu ~ (E_nu)^-2.  This is not a surprise;  most of us thought that the -2 index was based on a simplified model which would not survive an encounter with data.

The second analysis, by Gary Binder,  also looked at the energy spectrum (with similar results), and also looked at the neutrino flavor ratio: how many of the neutrinos are electron neutrinos (nu_e), vs. muon neutrinos (nu_mu)  vs. tau neutrinos (nu_tau) . It is available on the Cornell preprint server, as arXiv:1502.03376.   It found a similar spectral index (as have other IceCube studies). 

The flavor ratio is somewhat tricky, in that there are three different types of neutrinos which interact via two topologies: long muon tracks from nu_mu, and roughly spherical showers, from nu_e and all flavor charged-current interactions.  In this energy range, 83% of nu_tau produce showers, while 17% of them include muons.  So, there is some ambiguity.  Gary presented his results as a triangle:
Each point in the triangle corresponds to a specific nu_e:nu_mu:nu_tau ratio; the fraction can be found by reading across to the right for the nu_tau fraction, downward to the right for the nu_tau fraction, and upward to the left for the nu_e fraction.  The four symbols in the legend correspond to four different models for neutrino production in a source: via pion decay to muons, via pion decay to muons which lose energy rapidly, via neutron decay, and via pion decay to muons which gain energy before they decay.  At the source, these models predict quite different flavor ratios.  However, the sources are very distant, and the neutrinos will oscillate during their trip, arriving at something much closer to an equal mix of flavors. 

The current analysis find a best fit indicated by the cross near the lower left.  However, the confidence levels (shown via colors) show that all four models can adequately fit the data.  However, the analysis does rule out non-standard models (not shown here, but discussed in the paper) such as some involving sterile (non-interacting) neutrinos.